Don't block on rpc calls in unit tests

Don't block on rpc calls in unit tests

While running unit tests I noticed that all of the test cases from
TestOvsNeutronAgent were taking just over 15 seconds.  This is because
it was waiting for the rpc call response timeout.  Since the tests
weren't actually failing, it's clear the code doesn't actually care
about the response anyway, so just set the messaging config to not
wait for the response.

On my dev box, this dropped test runtime for this file from 1024
seconds to 3 seconds, which is an obvious *HUGE* improvement.

Someone pointed out that an earlier change
(086496bfc45e01cd2905a074d526a7d513bf4ec2) intended to make calls
return immediately for *all* tests, but removing use of RpcProxy
caused this to stop working.

Change-Id: Id07c1d1727a3f4087f0a97681388fb5d69740f8d